Use a string and 1x2 stakes to lay out the trench.
How far extend the end of the gutter.
The back edge of the gutter should tuck up behind the dripedge if you have it otherwise the edge of the shingles should extend well over the back half of the gutter so the drips cannot get behind it and rot the fascia board.
Pound in two stakes 4 feet from the house and directly in front of each downspout.
Place other stakes spaced 8 feet apart in between.
Tie the string to the downspout then stretch it out and around all the stakes.
Hold a wooden block flat across the outer end of the connector and tap the block with the hammer to fit the connector onto the end of the existing gutter.
To be safe aim for a 4 foot minimum extension away from a downspout.
This one s a popular solution for a few good reasons.
3 measure the distance from the open end.